Acrylic acid is industrially important as a starting material for various synthetic resins, paints, plasticizing agents and the like, and is generally produced by two-stage oxidation method, in which propylene is catalytically oxidized at vapor phase to yield mainly acrolein, and the acrolein is successively catalytically oxidized at vapor phase to yield acrylic acid. In recent years, importance of acrylic acid as a starting material of water absorbent resins is constantly increasing. To cope with such increasing demand, a simple and general solution is to raise the productivity of acrylic acid by increasing the load of the starting material at the steady state (standard operating conditions) of the catalytic vapor-phase oxidation. Hence, also in the vapor-phase oxidation of propylene, which is the first stage reaction in the acrylic acid production by catalytic vapor-phase oxidation of propylene, similarly the operation under high starting material load is necessary.
The catalytic vapor-phase oxidation of propylene, however, is exothermic, and the calorific value also increases when the propylene load as the starting material is increased. Besides, during the period of start-up from the non-reacting condition up to immediately after attaining the prescribed reaction conditions, the catalytic activity is unstable, and when the propylene load is rapidly increased at the start-up, abnormal heat generation in the catalyst layer(s) is apt to occur to give rise to local heat-generating sites (hot spots), inviting in consequence reduction in the acrolein and acrylic acid yield due to the high temperature reaction and deterioration of the catalyst which is exposed to the high temperature. Such problems become even more serious when the vapor-phase oxidation is carried out under high load condition.
Thus, in the method of producing acrolein and acrylic acid by catalytic vapor-phase oxidation of propylene with molecular oxygen, a process enabling the production with higher stability and higher productivity or yield is in demand, and a number of proposals have been made also about contrivances for the start-up.
For example, the following patent documents 1, 2 and 3 disclose methods in which the supply amount per unit time of the starting material is kept low for a fixed period at the start-up stage of the reaction.
[Prior Art]
[Patent Documents]
                [Patent document 1] JP2003-12589A        [Patent document 2] JP2005-336085A        [Patent document 3] JP2007-502254T        